Network Working Group Y. Rekhter, Editor
Request for Comments: 1266 T.J. Watson Research Center, IBM Corp.
October 1991
Experience with the BGP Protocol
1. Status of this Memo.
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
2. Introduction.
The purpose of this memo is to document how the requirements for
advancing a routing protocol to Draft Standard have been satisfied by
Border Gateway Protocol (BGP). This report documents experience with
BGP. This is the second of two reports on the BGP protocol. As
required by the Internet Activities Board (IAB) and the Internet
Engineering Steering Group (IESG), the first report will present a
performance analysis of the BGP protocol.
The remaining sections of this memo document how BGP satisfies
General Requirements specified in Section 3.0, as well as
Requirements for Draft Standard specified in Section 5.0 of the
"Internet Routing Protocol Standardization Criteria" document [1].
This report is based on the work of Dennis Ferguson (University of
Toronto), Susan Hares (MERIT/NSFNET), and Jessica Yu (MERIT/NSFNET).
Details of their work were presented at the Twentieth IETF meeting
(March 11-15, 1991, St. Louis) and are available from the IETF
Proceedings.
Please send comments to iwg@rice.edu.
3. Acknowledgements.
The BGP protocol has been developed by the IWG/BGP Working Group of
the Internet Engineering Task Force. We would like to express our
deepest thanks to Guy Almes (Rice University) who was the previous
chairman of the IWG Working Group. We also like to explicitly thank
Bob Hinden (BBN) for the review of this document as well as his
constructive and valuable comments.
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4. Documentation.
BGP is an inter-autonomous system routing protocol designed for the
TCP/IP internets. Version 1 of the BGP protocol was published in RFC
1105. Since then BGP Versions 2 and 3 have been developed. Version 2
was documented in RFC 1163. Version 3 is documented in [3]. The
changes between versions 1, 2 and 3 are explained in Appendix 3 of
[3]. Most of the functionality that was present in the Version 1 is
present in the Version 2 and 3. Changes between Version 1 and
Version 2 affect mostly the format of the BGP messages. Changes
between Version 2 and Version 3 are quite minor.
BGP Version 2 removed from the protocol the concept of "up", "down",
and "horizontal" relations between autonomous systems that were
present in the Version 1. BGP Version 2 introduced the concept of
path attributes. In addition, BGP Version 2 clarified parts of the
protocol that were "underspecified". BGP Version 3 lifted some of
the restrictions on the use of the NEXT_HOP path attribute, and added
the BGP Identifier field to the BGP OPEN message. It also clarifies
the procedure for distributing BGP routes between the BGP speakers
within an autonomous system. Possible applications of BGP in the
Internet are documented in [2].
The BGP protocol was developed by the IWG/BGP Working Group of the
Internet Engineering Task Force. This Working Group has a mailing
list, iwg@rice.edu, where discussions of protocol features and
operation are held. The IWG/BGP Working Group meets regularly during
the quarterly Internet Engineering Task Force conferences. Reports of
these meetings are published in the IETF's Proceedings.
5. MIB
A BGP Management Information Base has been published [4]. The MIB
was written by Steve Willis (swillis@wellfleet.com) and John Burruss
(jburruss@wellfleet.com).
Apart from a few system variables, the BGP MIB is broken into two
tables: the BGP Peer Table and the BGP Received Path Attribute Table.
The Peer Table reflects information about BGP peer connections, such
as their state and current activity. The Received Path Attribute
Table contains all attributes received from all peers before local
routing policy has been applied. The actual attributes used in
determining a route are a subset of the received attribute table.
The BGP MIB is quite small. It contains total of 27 objects.
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6. Security architecture.
BGP provides flexible and extendible mechanism for authentication and
security. The mechanism allows to support schemes with various degree
of complexity. All BGP sessions are authenticated based on the BGP
Identifier of a peer. In addition, all BGP sessions are authenticated
based on the autonomous system number advertised by a peer. As part
of the BGP authentication mechanism, the protocol allows to carry
encrypted digital signature in every BGP message. All authentication
failures result in sending the NOTIFICATION messages and immediate
termination of the BGP connection.
Since BGP runs over TCP and IP, BGP's authentication scheme may be
augmented by any authentication or security mechanism provided by
either TCP or IP.
7. Implementations.
There are multiple interoperable implementations of BGP currently
available. This section gives a brief overview of the three
completely independent implementations that are currently used in the
operational Internet. They are:
- cisco. This implementation was wholly developed by cisco.
It runs on the proprietary operating system used by the
cisco routers. Consult Kirk Lougheed (lougheed@cisco.com)
for more details.
- "gated". This implementation was developed wholly by Jeff
Honig (jch@risci.cit.cornell.edu) and Dennis Ferguson
(dennis@CAnet.CA). It runs on a variety of operating systems
(4.3 BSD, AIX, etc...). It is the only available public domain
code for BGP. Consult Jeff Honig or Dennis Ferguson for more
details.
- NSFNET. This implementation was developed wholly by Yakov
Rekhter (yakov@watson.ibm.com). It runs on the T1 NSFNET
Backbone and T3 NSFNET Backbone. Consult Yakov Rekhter for
more details.
To facilitate efficient BGP implementations, and avoid commonly made
mistakes, the implementation experience with BGP in "gated" was
documented as part of RFC 1164. Implementors are strongly encouraged
to follow the implementation suggestions outlined in that document.
Experience with implementing BGP showed that the protocol is
relatively simple to implement. On the average BGP implementation
takes about 1 man/month effort.
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Note that, as required by the IAB/IESG for Draft Standard status,
there are multiple interoperable completely independent
implementations, namely those from cisco, "gated", and IBM.
8. Operational experience.
This section discusses operational experience with BGP.
BGP has been used in the production environment since 1989. This use
involves all three implementations listed above. Production use of
BGP includes utilization of all significant features of the protocol.
The present production environment, where BGP is used as the inter-
autonomous system routing protocol, is highly heterogeneous. In
terms of the link bandwidth it varies from 56 Kbits/sec to 45
Mbits/sec. In terms of the actual routes that run BGP it ranges from
a relatively slow performance PC/RT to a very high performance
RS/6000, and includes both the special purpose routers (cisco) and
the general purpose workstations running UNIX. In terms of the actual
topologies it varies from a very sparse (spanning tree or a ring of
CA*Net) to a quite dense (T1 or T3 NSFNET Backbones).
At the time of this writing BGP is used as an inter-autonomous system
routing protocol between the following autonomous systems: CA*Net, T1
NSFNET Backbone, T3 NSFNET Backbone, T3 NSFNET Test Network, CICNET,
MERIT, and PSC. Within CA*Net there are 10 border routers
participating in BGP. Within T1 NSFNET Backbone there are 20 border
routers participating in BGP. Within T3 NSFNET Backbone there are 15
border routers participating in BGP. Within T3 NSFNET Test Network
there are 7 border routers participating in BGP. Within CICNET there
are 2 border routers participating in BGP. Within MERIT there is 1
border router participating in BGP. Within PSC there is 1 router
participating in BGP. All together there are 56 border routers
spanning 7 autonomous systems that are running BGP. Out of these, 49
border routers that span 6 autonomous systems are part of the
operational Internet.
BGP is used both for the exchange of routing information between a
transit and a stub autonomous system, and for the exchange of routing
information between multiple transit autonomous systems. It covers
both the Backbones (CA*Net, T1 NSFNET Backbone, T3 NSFNET Backbone),
and the Regional Networks (PSC, MERIT).
Within CA*Net, T3 NSFNET Backbone, and T3 NSFNET Test Network BGP is
used as the exclusive carrier of the exterior routing information
both between the autonomous systems that correspond to the above
networks, and with the autonomous system of each network. At the time
of this writing within the T1 NSFNET Backbone BGP is used together
with the NSFNET Backbone Interior Routing Protocol to carry the
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exterior routing information. T1 NSFNET Backbone is in the process of
moving toward carrying the exterior routing information exclusively
by BGP. The full set of exterior routes that is carried by BGP is
well over 2,000 networks.
Operational experience described above involved multi-vendor
deployment (cisco, "gated", and NSFNET).
Specific details of the operational experience with BGP in the NSFNET
were presented at the Twentieth IETF meeting (March 11-15, 1991, St.
Louis) by Susan Hares (MERIT/NSFNET). Specific details of the
operational experience with BGP in the CA*Net were presented at the
Twentieth IETF meeting (March 11-15, 1991, St. Louis) by Dennis
Ferguson (University of Toronto). Both of these presentations are
available in the IETF Proceedings.
Operational experience with BGP exercised all basic features of the
protocol, including the authentication and routing loop suppression.
Bandwidth consumed by BGP has been measured at the interconnection
points between CA*Net and T1 NSFNET Backbone. The results of these
measurements were presented by Dennis Ferguson during the last IETF,
and are available from the IETF Proceedings. These results showed
clear superiority of BGP as compared with EGP in the area of
bandwidth consumed by the protocol. Observations on the CA*Net by
Dennis Ferguson, and on the T1 NSFNET Backbone by Susan Hares
confirmed clear superiority of BGP as compared with EGP in the area
of CPU requirements.
9. Using TCP as a transport for BGP.
9.1. Introduction.
On multiple occasions some members of IETF expressed concern about
using TCP as a transport protocol for BGP. In this section we examine
the use of TCP for BGP in terms of:
- real versus perceived problems
- offer potential solutions to real problems
- perspective on the convergence problem
- conclusions
BGP is based on the incremental updates. This is done intentionally
to conserve the CPU and bandwidth requirements. Extensive operational
experience with BGP in the Internet showed that indeed the use of the
incremental updates allows significant saving both in terms of the
CPU utilization and bandwidth consumption. However, to operate
correctly the incremental updates must be exchanged over a reliable
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transport. BGP uses TCP as such transport. It had been suggested
that another transport protocol would be more suitable for BGP.
9.2. Examination of Problems - Real and "perceived".
Extensive operational experience with BGP in the Internet showed that
the only real problem that was attributed to BGP in general, and the
use of TCP as the transport for BGP in particular, was its slow
convergence in presence of congestion. This problem was experienced
in CA*Net. As we mentioned before, CA*Net is composed of 10 routers
that form a ring. The routers are connected by 56 Kbits/sec links.
All links are heavily utilized and are often congested. Experience
with BGP in CA*Net showed that unless special measures are taken, the
protocol may exhibit slow convergence when BGP information is passed
over the slow speed (56 Kbits/sec) congested links. This is because a
large percentage of packets carrying BGP information are being
dropped due to congestion. Therefore, there are three inter-related
problems: congestion, packet drops, and the resulting slow
convergence of routing under congestion and packet drops.
Observe, that any transport protocol used by BGP would have
difficulty preventing packets from being dropped under congestion,
since it has no direct control over the routers that drop the
packets, and the congestion has nothing to do with the BGP traffic.
Therefore, since BGP is not the cause of congestion, and cannot
directly influence dropping at the routers, replacing TCP (as the BGP
transport) with another transport protocol would have no effect on
packets being dropped due to congestion. We think that once a network
is congested, packets will be dropped (regardless of whether these
packets carry BGP or any other information), unless special measures
outside of BGP in general, and the transport protocol used by BGP in
particular, are taken.
If packets carrying routing information are lost, any distributed
routing protocol will exhibit slow convergence. If quick convergence
is viewed as important for a routing within a network, special
measures to minimize the loss of packets that carry routing
information must be taken. The next section suggests some possible
methods.
9.3. Solutions to the problem.
Two possible measures could be taken to reduce the drop of BGP
packets which slows convergence of routing:
1) alleviate the congestion
2) reduce the percentage of BGP packets that are dropped due
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to congestion by marking BGP packets and setting policies to
routers to try not to drop BGP packets
Alleviating the network congestion is a subject outside the control
of BGP, and will not be discussed in this paper.
Operational experience with BGP in CA*Net shows that reducing the
percentage of BGP packets dropped due to congestion by marking them,
and setting policies to routers to try not to drop BGP packets
completely solves the problem of slow convergence in presence of
congestion.
The BGP packets can be marked (explicitly or implicitly) by the
following three methods:
a) by means of IP precedence (Internetwork Control)
b) by using a well-known TCP port number
c) by identifying packets by just source or destination IP
address.
Appendix 4 of the BGP protocol specification, RFC 1163, recommends
the use of IP precedence (Internetwork Control) because the
precedence provides a well-defined mechanism to mark BGP packets.
The method of a well-known TCP port number to identify packets is
similar to the one that was used by Dave Mills in the NSFNET Phase I.
Dave Mills identified Telnet traffic by a well known TCP port number,
and gave it priority over the rest of the traffic. CA*Net identified
BGP traffic based on it's source and destination IP address. Packets
receive a priority if either the source or the destination IP address
belongs to CA*Net.
If packets that carry the routing information are being dropped
(because of congestion), one also may ask about how does a particular
routing protocol react to such an event. In the case of BGP the
packets are retransmitted using the TCP retransmission mechanism. It
seems plausible that being more aggressive in terms of the
retransmission should have positive effect on the convergence. This
can be done completely within TCP by adjusting the TCP retransmission
timers. However, we would like to point out that the change in the
retransmission strategy should not be viewed as a cure for the
problem, since the root of the problem lies in the way how packets
that carry the BGP information are handled within a congested
network, and not in how frequently the lost packets are
retransmitted.
It should also be pointed out that the local system can control the
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amount of data to be retransmitted (in case of a congestion or
losses) by adjusting the TCP Window size. That allows to control the
amount of potentially obsolete data that has to be retransmitted.
9.4. Perspective on the Convergence Problem.
To put the convergence problem in a proper perspective, we'd like to
point out that much of the Internet now uses EGP at AS borders,
ensuring that routing changes cannot be guaranteed to propagate
between ASes in less than a few minutes. It would take huge amount of
congestion to slow BGP to this pace. Additionally, the problems of
EGP in the face of packet loss are well known and far exceed any
imaginable problem BGP/TCP might ever suffer. Therefore, the worst
case behavior of BGP is about the same as the steady case behavior of
EGP.
Within an AS the speed of convergence of the AS's IGP in the face of
congestion is of far greater concern than the propagation speed of
BGP, and indeed avoiding loss of packets carrying IGP, and a more
aggressive transport is similarly of much greater importance for an
IGP than for BGP.
The issue of BGP convergence is of exaggerated importance to CA*Net
since CA*Net carries no information about external routes in its IGP.
CA*Net uses BGP to transfer external routes for use in computing
internal routes through the CA*Net network. The reason CA*Net does
this has nothing to do with BGP. Under more ordinary circumstances an
IGP carries external routing information for use in computing
internal routes. CA*Net shows that BGP can work under extreme stress.
However, it's results should not be taken as the norm since most
networks will use BGP in a different (and less stressful)
configuration, where information about external routes will be
carried by an IGP.
9.5. Conclusion.
The extensive operational experience with BGP showed that the only
problem attributed to BGP was the slow convergence problem in
presence of congestion. We demonstrated that this problem has
nothing to do with BGP in general, or with TCP as the BGP transport
in particular, but is directly related to the way how packets that
carry routing information are handled within a congested network. The
document suggests possible ways of solving the problem. We would
like to point out that the issue of convergence in presence of
congested network is important to all distributed routing protocol,
and not just to BGP. Therefore, we recommend that every routing
protocol (whether it is intra-autonomous system or inter-autonomous
system) should clearly specify how its behavior is affected by the
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congestion in the networks, and what are the possible mechanisms to
avoid the negative effect of congestion (if any).
10. Bibliography.
[1] Hinden, B., "Internet Routing Protocol Standardization Criteria",
RFC 1264, BBN, October 1991.
[2] Rekhter, Y., and P. Gross, "Application of the Border Gateway
Protocol in the Internet", RFC 1268, T.J. Watson Research Center,
IBM Corp., ANS, October 1991.
[3] Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol 3 (BGP-
3)", RFC 1267, cisco Systems, T.J. Watson Research Center, IBM
Corp., October 1991.
[4] Willis, S., and J. Burruss, "Definitions of Managed Objects for
the Border Gateway Protocol (Version 3)", RFC 1269, Wellfleet
Communications Inc., October 1991.
Security Considerations
Security issues are discussed in section 6.
Author's Address
Yakov Rekhter
T.J. Watson Research Center IBM Corporation
P.O. Box 218
Yorktown Heights, NY 10598
Phone: (914) 945-3896
EMail: yakov@watson.ibm.com
IETF BGP WG mailing list: iwg@rice.edu
To be added: iwg-request@rice.edu
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